Well, The Strategy is Simple (This is My Gengar of Pokémon Quetzal With the only difference that I use Gengarianite instead of Focus Sash)
GEEENGAR! (M) @ Focus Sash
EVs: 252 SpA 252 Spe And 4 HP
IVs: 0 Atk
Nature: Modest or Timid (Modest is for more Offensive)
Ability: Cursed Body (unfortunately)
-Hypnosis
-Nasty Plot
-Shadow Ball
-Dream Eater, Sludge Wave or Or Anything of Your Preference
Well, here the strategy is clear, use Hypnosis to make your enemy sleep like a baby, then use a Nasty Plot to make your Gengar nice and strong, from there it's up to you, you can use Dream Eater for healing and damage, Shadow Ball for STAB, Agility in the fourth slot or some other Buff move to Buff yourself more, In the fourth slot, you can also put some Move for Gengar that offers coverage against any weakness that your team has (For example, Thundebolt if your team is weak against Water Pokes) and lastly, you can be greedy or use more than one Nasty Plot (like two) that's why this Gengar has Focus Sash By the way, in case you use more than one Nasty Plot, if the Enemy wakes up, the Focus Sash will protect you from Hit Kills, anyway I LOVE this Set for Gengar because it's Versatile, and can be used for any team, anyway, bye :3